M.Sc. Part I 1.3 INFRARED SPECTROPHOTOMETRY PPT.ppsx

INFRARED SPECTROPHOTOMETRY

1. 1
Infraredspectroscopy

2. INFRARED ABSORPTION SPECTROSCOPY
Syllabus
Instrumentation,
FTIR,
Advantages of FTIR,
Applications of IR,
Qualitative and quantitative analysis,
Advantages and limitations of quantitative IR
methods.
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3. INTRODUCTION:
•InfraredSpectroscopyisapowerfultoolfor
identifyingpureorganicandinorganic
compoundswiththeexceptionsoffew
molecularcompoundssuchasO
2,N
2,andCl
2.
Attemperatureaboveabsolutezero,allatoms
inmoleculesareincontinuousvibrationwith
respecttoeachother.Whenthefrequencyof
specificvibrationsareequaltothefrequency
oftheIRradiationdirectedonthemolecule,
themoleculesabsorbIRradiation.
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4. Infrared region
Near-infrared
(overtone region)
0.8-2.5 um
(12 500-4000 cm -1);
Middle infrared
(vibration-rotation
region)
2.5-50 um
(4000-200 cm -1);
Far-infrared
(rotation region)
50-1000 um
(200-10 cm-1).
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5. A molecule when absorb IR
radiation it undergoes
vibrations due to
Bonds may stretch
back and forth
Bonds may rotate
Bonds may bend
and flex
Groups on atoms
may “wag” and
“scissor
The molecule may
rotate and vibrate
in other ways
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6. What is vibration
ForaC-Cbondwithabond
lengthof154pm,the
variationisabout10pm.
ForC-C-Cbondanglea
changeof4
o
istypical.
Thismovesacarbonatom
about10pm.
4
o 10
pm
10 pm
154 pm
stretching vibration
bending vibration
6

7. ForaC-Cbondwithabond
lengthof154pm,the
variationisabout10pm.
Bond length 154 pm,
10 pm.
7

8. C
C
C
4
o
10 pm
8
ForC-C-Cbondangleachangeof4
o
istypical.Thismovesacarbonatomabout
10pm.

9. How does the mass influence the vibration?
H
2
I
2
MM =2 g/mole
MM =254 g/mole
The greater the mass -the lower the wave number
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10. 10
Instrumentation
Radiation
source
Sample
handling
DetectorRecorder

11. Radiation source:
•Thevarioussourcesofradiationsourceareused
inI.R.
•NernstGlower:ANernstglowerisheatedto
1500-2000
o
ctogetinfraredradiation.Nernst
glowerisarodcontainingamixtureof
zirchonium,yttriumanderbiumoxides.Platinum
leadsaresealedintotwoendsofrods.The
NernstGloweris2cmlongandhasdiameterof
1.5mm.ItsIRoutputdecreaseswith
temperature.
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12. GloberRod:
•TheGloberrodissiliconcarbiderod(SiC)which
onheatingto1500
0
CemitsIRradiation.Its
disadvantageisthatSiCgetseasilyoxidized
henceithasshorterlifethanNernstGlower;
HoweveritsadvantageisthatItsIRoutput
increaseswithincreaseintemperature.
12

13. Carbon dioxide Laser Source:
•For measurements in the middle-infrared region, 2.5-
50 PM, there are several differences between the
instruments used for UV/visible Spectrophotometry
and those designed for infrared determinations. These
changes are mainly dictated by the fact that glass and
quartz absorb strongly in the infrared region and
photomultipliers are insensitive to the radiation. Front-
surfaced mirrors are largely employed to avoid the
necessity of radiation passing through glass or quartz
layers as reflection from metallic surfaces is generally
very efficient in the infrared region. But absorption
cells and windows must be fabricated from infrared
transparent materials.
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14. Sample Cells:
•2 a) Sample Cells:
•The sample cells of metal halide like NaCland
KBrare generally used.
•Quartz and glass are not used because it
absorbs strongly in IR region.
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15. 2 b) Sample Handling System:
•Thesolventusedforpreparationofsample
solutionshouldnotabsorbIRradiation.
•Allsolventsaredriedtoremovemoisturebefore
used.
•SolventscommonlyusedinIRstudiesarehexane,
Chloroform,carbontetrachloride,Dioxane,
carbondisulphide,ethanol,methanolandinrare
casebenzene.
•Thesolutionconcentrationrangefrom0.1%to
10%.
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16. Sample handling and preparation
•Gaseous sample: Gaseous sample are taken in
10 cm long cells provided with NaCl windows
which are transparent to IR radiation.
•Liquid samplesare pressed between two NaCl
plates which are held together by capillary
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17. Sampling cuvettes for a) gaseous and b) liquid samples
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Gaseous Sample Liquid samples

18. For Solid samples following two methods are
used .
KBrPalletMethod:Analkalihalide(ARGrade)KBr
palletispreparedbygrinding1mgofsamplewith100-
200mgsofKBr.
Thepalletisdriedtoremovemoistureandpressed
underhighpressureintosmalltransparentdiscof10
mmandthickness1-2mm.
Thedisccandirectlyusedinsamplingareaofthe
spectrophotometer.
MullMethod:Amullispreparedbygrinding5mgsof
sampleofsolidsamplewithfewdropsofmullingoil.
Themullingoilcommonlyusedisishighboiling
petroleumoilcallednujol.
ThemullisthenpressedbetweentwoNaClwindows.
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19. Solid sample:
Die assembly for making KBrpellets
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20. Detector:
•The various detectors used in IR spectroscopy
are as ;
Thermocouple
A bolometer
The Golaypneumatic detector
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21. The Thermocouple:
•TheThermocouple:Itismadebyweldingtogether
twowiresofmetals1and2insuchamannerthata
segmentofmetal1isconnectedtotwoterminalwires
ofmetal2.Onejunctionbetweenmetals1and2is
heatedbytheinfraredbeam,andtheotherjunctionis
keptatconstanttemperature:smallchangesin
ambienttemperaturearethusminimized.Toavoid
lossesofenergybyconvection,thecouplesare
enclosedinanevacuatedvesselwithawindow
transparenttoinfraredradiation.Themetallic
junctionsarealsocoveredwithablackdepositto
decreasereflectionoftheincidentbeam
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22. Thermocouple:
22

23. A Bolometer:-
•Abolometerisessentiallyathinblackenedplatinum
stripinanevacuatedglassvesselwithawindow
transparenttotheinfraredrays:itisconnectedasone
armofaWheatstonebridge,andanyradiation
absorbedraisesthetemperatureofthestripand
changesitsresistance.Twoidenticalelementsare
usuallyplacedintheoppositearmsofabridge;oneof
theelementsisinthepathoftheinfraredbeamand
theothercompensatesforvariationsinambient
temperature.Boththeabovereceptorsgiveavery
smalldirectcurrent,whichmaybeamplifiedbyspecial
methodstodrivearecorder.
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24. The GolayPneumatic Detector
Itissometimesused,consistsofagas-filledchamberwhich
undergoesapressurerisewhenheatedbyradiantenergy.
Smallpressurechangescausedeflectionsofonewallofthechamber,
thismovablewallalsofunctionsasamirrorandreflectsalightbeam
directeduponittoaphotocell,theamountoflightreflectedbearing
adirectrelationtothegas-chamberexpansion,andhencetothe
radiantenergyofthelightfromthemonochromator.
Thisdetectorrespondstothetotallightenergyreceivedasdistinct
fromenergyreceivedperunitarea(thermocouplesandbolometers).
Wheninfraredradiationisincidentonthedetectorthereisachange
inpolarisationwhichcanbeemployedtoproduceanelectricalsignal.
Thedetectorwillonlyproduceasignalwhentheintensityofthe
incidentradiationchanges.
ThesedetectorsareofespecialvalueinFT-IRwhererapidresponse
timesareneededandforthispurposetheyusedeuteriumtriglycine
sulphateasthedetectingmediuminanevacuatedchamber.
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25. •InFTIRinstruments,Instrumentscollectthe
responseatallthewavelengthssimultaneously.In
thesemultiplexinstrumentsthemonochromatoris
replacedbyanInterferometerandtheresponseof
thesampletothewholerangeofIRradiationis
suitablymodulatedandcollectedinthetimedomain.
•ThesearethendecodedusingtheFourier
transformation.ThereforeitisnamedtheFourier
transforminfraredspectrometer(FT-IR).
•ThebasicinstrumentdesignforanFT-IRinstrumentis
quitesimple.Aschematicdiagramshowingdifferent
componentsisgiveninFig.
•TheIRradiationfromthesourceisfirstdirectedinto
aninterferometerfromwhereitispassedthrough
thesamplecompartmentandthenitreachestothe
detector.
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26. 26
FT IR Instruments

27. Interferometers as wavelength
selector In FTIR Spectroscopy:
•Adevicethatallowsallwavelengthsoflighttobemeasured
simultaneously,eliminatingtheneedforawavelengthselector
interferometersimultaneouslyallowssourceradiationofallwavelengths
toreachthedetector.Radiationfromthesourceisfocusedonabeam
splitterthattransmitshalfoftheradiationtoafixedmirror,while
reflectingtheotherhalftoamovablemirror.
•Theradiationrecombinesatthebeamsplitter,whereconstructiveand
destructiveinterferencedetermines,foreachwavelength,theintensityof
lightreachingthedetector.
•Asthemovingmirrorchangesposition,thewavelengthsoflight
experiencingmaximumconstructiveinterferenceandmaximum
destructiveinterferencealsochanges.
•Thesignalatthedetectorshowsintensityasafunctionofthemoving
mirror’sposition,expressedinunitsofdistanceortime.Theresultis
calledaninterferogram,oratimedomainspectrum.Thetimedomain
spectrumisconvertedmathematically,byaprocesscalledaFourier
transform,tothenormalspectrum(alsocalledafrequencydomain
spectrum)ofintensityasafunctionoftheradiation’senergy
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28. 28
QualitativeApplications
Themostimportantqualitativeapplicationofthemid-IR
spectrometryisinthedeterminationofthestructuresof
organicandbiochemicalspecies.
Itisachievedinanempiricalmethodwhereinthesignals
intheIRspectraareidentifiedwiththehelpof
correlationtables(brieflydiscussedbelow).Besides
structuredetermination,weshalldiscusshowtheIR
spectracanbeusedinestablishingtheidentityofa
molecule?Andalso,howdoesithelpinmonitoringthe
progressofareaction?

29. Structure Elucidation of Organic
Molecules by IR Spectra
•For structure determination of organic molecules, the
infrared spectrum can be broadly divided into two regions.
•The region spanning from 3600 to 1200 1 cm− is called the
functional group region
•The region that includes all frequencies below 1200 cm-1
is called the fingerprint region.
•The two regions put together are important in the
determination of the identity of a molecule.
•It is almost impossible to assign all the possible frequencies
observed in the IR spectrum. However, identification of the
•characteristic features in the functional group region that
includes stretching vibrations, of typical functional groups
found in organic molecules, is quite important.
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30. Quantitative analysis :
•Infrared spectra are recorded using either or
both absorbance and percentage transmission
just as they are in visible/ultraviolet electronic
spectra, and the Beer-Lambert relationship:
•for a mixture of compounds the observed
absorbance at a particular wavelength (or
frequency) will be the sum of the absorbances
for the individual constituents of the mixture
at the wavelength:
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31. 2) Calibration Curve Method:
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32. 32

33. Standard addition methods:
Thesearenotwidelyappliedinquantitativeinfrared
spectrophotometry,beinglimitedtodeterminationsoflow
concentrationcomponentsinmulticomponentmixtures.
Thesolutionsaremadefromaseriesofincreasing
concentrationsofthepureanalyte(similartoanormal
calibrationgraphsetofconcentrations)buttoeachis
addedaconstant,knownamountofthesamplecontaining
theunknownconcentration.Allthesolutionsaredilutedto
afixedvolumeandtheirabsorbance’smeasuredinafixed-
path-lengthcellbyscanningoverthechosenabsorption
band.
Aplotoftheabsorbanceagainsttheconcentrationofthe
pureanalytedoesnotpassthroughzeroasallthe
absorbancevaluesareenhancedbyanequalamountdue
tothepresenceoftheunknownconcentrationintheadded
sample.Extrapolationofthegraphbacktotheabscissa(the
horizontalaxis)givestheconcentrationoftheunknownas
anegativevalue(Graph)
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34. 34

35. Other Miscellaneous applications:
•i)IRspectroscopyiswidelyusedinbothresearchandindustry
asasimpleandreliabletechniqueformeasurement,quality
controlanddynamicmeasurement.
•ii)IRspectroscopyhasbeenhighlysuccessfulforapplications
inbothorganicandinorganicchemistry.
•iii)Bymeasuringataspecificfrequencyovertime,changesin
thecharacterorquantityofaparticularbondcanbe
measured.Thisisespeciallyusefulinmeasuringthedegreeof
polymerizationinpolymermanufacture.
•iv)IRspectroscopyisusefulforidentifyingsubstancesand
confirmingtheiridentity.Thereforeitalsohasaforensic
purpose:withtheuseofIRspectroscopyalcohol,drugs,fibers
andpaintcouldbeanalyzed.
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36. •v)IRspectroscopyhasalsobeensuccessfully
utilizedinthefieldofsemiconductor
microelectronics:
•forexample,thistechniquecanbeappliedto
semiconductorslikesilicon,galliumarsenide,
galliumnitride,zincselenide,amorphous
silicon,andsiliconnitride.
•vi)Techniqueshavebeendevelopedtoassess
thequalityoftea-leavesusingIRspectroscopy.
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37. Advantages of quantitative IR methods.
1)Anysampleinanystatecanbestudied.
2)Liquidsamplesolutions,pastespowders,films
fibres,gasesandsurfacescanbeexaminedwith
judiouschoiceofsamplingtechnique.
3)ApplicationofIRspectroscopyvariesfrom
laboratorytolaboratorytoother.
4)QuanitativeanalysisisbasedonBeersLamberts
Law.
5)Apparentdeviationsarisesfromeitherchemical
orinstrumentaldefects.
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38. 5)Thebaselinetechniqueinvolvesselectionof
anabsorptionbandofthesubstanceunder
analysis.
6)Thetransmittancespeedismeasuredat
pointofmaximumabsorption.
7)ThevalueofPoisplottedagainst
concentration
8)Manypossibleerrorareeliminatedbybase
linetechnique.
9)Thesamecellisusedforalldetermination
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39. limitations of quantitative IR methods.
1)Molecularweightofsubstanceisnot
determinedbythistechnique.
2)Itisfrequentlynon-adherencetoBeersLaw
ofComplexityspectra.
3)Narrownessofspectraandeffectofstrayray
radiationsmakethemeasurementsof
absorbanceuponslitwidthandwavelenght
setting.
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40. 4)GenerallyIRspectroscopydoesnotprovide
informationoftherelativepositionsof
differentfunctionalgroupsinamolecule.
5)FromsingleIRspectrumofanunknown
substance,Itisnotpossibletoknowwhether
purecompoundormixtureofcompound,For
Ex.Mix.Ofparafinandalcoholwillgivethe
sameIRspectraashighmolecularweightof
alcohol.
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